1. Field of the Invention
The present invention relates to a display device employing light-emitting elements using an organic compound, and more particularly, to a display device using organic electroluminescence (EL) elements that emit light when an electric field is applied to a thin film comprised of an organic compound.
2. Description of the Related Art
Organic EL elements (organic light-emitting elements or organic electroluminescence elements) have been actively researched and developed in recent years. When a display device utilizing any one of such organic EL elements is used in an outdoor environment, ambient light is incident on the organic EL element through a transparent electrode, is reflected on a metal electrode inside the element, is emitted from the transparent electrode, and returns to the outside of the element again. Accordingly, for example, when a display device is used out of doors, the reflection of ambient light or the reflection of outside scenery occurs, thereby leading to a reduction in contrast or the deterioration of image quality.
In view of the foregoing, Japanese Patent Application Laid-Open No. H09-127885 discloses, for example, on page 4, lines 61-66 and in FIG. 2, a method of preventing a reduction in contrast or the deterioration of image quality by disposing a circularly polarizing plate on a display surface of a display device.
Further, Japanese Patent Application Laid-Open No. 2002-373776 discloses, for example, on page 3, lines 64-82 and in FIG. 2, a method of reducing an influence of the reflection of ambient light by using a combination of a light-absorbing filter and an optical resonator structure.
Moreover, Japanese Patent Application Laid-Open No. 2004-164890 discloses, for example, on page 4, lines 14-20 and FIG. 1, a monochromatic element in which a reduction in reflection of ambient light is achieved by using a quenching effect due to optical interference and a display device in which an influence of the reflection of ambient light is reduced by using a quenching effect due to optical interference and an optical filter such as a color filter in combination.
Besides, WO 2004/044998 discloses, for example, on page 4, lines 9-13 and in FIG. 1, a monochromatic element in which a reduction in reflection of ambient light is achieved with the aid of a quenching effect due to optical interference.
However, with the method disclosed in Japanese Patent Application Laid-Open No. H09-127885, although the suppressing effect on the reflection of ambient light is large, light emitted from an organic EL element as well as ambient light is quenched, so that the resultant luminance is 50% or less. Accordingly, when an attempt to obtain a desired luminance is to be made, the organic EL element needs to generate luminance which is twice or more the desired luminance, with the result that the power consumption increases. In addition, a high load is applied to the element, so that there is posed a problem such as a reduction in the lifetime of the element.
Further, the method disclosed in Japanese Patent Application Laid-Open No. 2002-373776 requires the step of forming the light-absorbing filter on a pixel, thereby causing a problem that a production process for a display device becomes complicated.
Moreover, with the elements and devices disclosed in Japanese Patent Application Laid-Open No. 2004-164890 and WO 2004/044998, although the ambient light reflectance in a wavelength region near the emission color is reduced, the ambient light reflectance in a wavelength region other than the wavelength region of the emission color increases. FIG. 20 is a graphical representation showing the ambient light reflection characteristics of a conventional organic EL element in which the reflection of ambient light is reduced by optical interference and which shows a red (R), a green (G), and a blue (B) emission colors. As shown in FIG. 20, a conventional ambient light quenching method according to an optical interference system has posed a problem that because in a light-emitting pixel portion of, for example, a blue or red emission color, the reflection of ambient light in a green color region cannot be sufficiently reduced, whereby the contrast of a display device is reduced. Accordingly, when the method is to be applied to a display device constituted by a plurality of organic EL elements of different emission colors, another optical means such as a color filter needs to be used in combination, thereby posing a problem that the constitution of the display device becomes complicated and the production cost of the device is high.